The Dynamic Albedo of Neutrons (DAN) is an active/passive neutron spectrometer that measures the abundance and depth distribution of H- and OH-bearing materials (e.g., adsorbed water, hydrated minerals) in a shallow layer (~1 m) of Mars' subsurface along the path of the MSL rover. In active mode, DAN measures the time decay curve (the "dynamic albedo") of the neutron flux from the subsurface induced by its pulsing 14 MeV neutron source. A detailed description of the DAN instrument and scientific investigation can be found in Litvak et al. (Astrobiology 8, p.605, 2008). The experiment is contributed by the Federal Space Agency of Russia.

The science objectives of the DAN instrument are as follows: 1) Detect and provide a quantitative estimation of the hydrogen in the subsurface throughout the surface mission; 2) Investigate the upper <0.5 m of the subsurface and determine the possible layering structure of hydrogen-bearing materials in the subsurface; 3) Track the variability of hydrogen content in the upper soil layer (~1 m) during the mission by periodic analysis; and 4) Track the variability of neutron radiation background (neutrons with energy < 100 keV) during the mission by periodic analysis.

The DAN instrument is expected to be used during rover traverses (e.g., during short stops at ~1 m intervals) and while the rover is parked. Short-duration (< 2 min) measurements will provide a rough estimate of the water-equivalent hydrogen distribution with an accuracy of ~1% by weight. Longer-duration (~30 min) measurements are necessary to derive the vertical distribution of water-equivalent hydrogen with an accuracy of 0.1-0.3% by weight.

Numerical simulations of the neutron count rate versus time (i.e., die-away curve) for the unshielded (left) and Cd-shielded (middle) detectors as a function of water abundance. The right panel shows the difference between the two count rates.